Advances in the semiconductor industry have reduced the size of transistors in integrated circuits (ICs) to 32 nanometers and smaller. The decrease in transistor sizes leads to decreases in power supply voltage to the transistors. As the power supply voltage has decreased, the threshold voltage of the transistors in the ICs has also decreased.
Lower threshold voltages are difficult to obtain in a conventional metal-oxide-semiconductor field-effect transistor (MOSFET). Indeed, as the threshold voltage is reduced the ratio of on current to off current (Ion/Ioff) also decreases. The on current refers to the current through the MOSFET when an applied gate voltage is above the threshold voltage, and the off current refers to current through the MOSFET when the applied gate voltage is below the threshold voltage.
The on current to off current ratio may be improved by using a tunneling field-effect transistor (TFET). The TFET takes advantage of band-to-band tunneling (BTBT) to increase the achievable on current (Ion), which permits further reductions in threshold voltage, power supply voltage, and transistor size. Unfortunately, forming the source, which has one doping type, and the drain, which has another doping type, in the TFET such that the source and drain are both suitably self-aligned with the gate stack is challenging.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.